John Roberts clerked for Justice Rehnquist in 1980-81. If he is confirmed, it appears the two men will serve on the Court together. Would this be the first time that two people, one of whom clerked for the other (on any court) served together on the Supreme Court?

[The Times ran an interesting article about the many other connections between the people involved in the nomination process.]

It has never happened on the Supreme Court, but has happened in the Circuit Courts, including currently on the Seventh Circuit where Terence Evans sits with his former clerk Diane Sykes (though Evans was on the district court when Sykes clerked for him). Anyone know how many times this has happened?

As has been said, it has never happened. A3G has gotten a response to this same question to this effect:

A well-informed reader offers this enlightening response: "Never. There have been four Supreme Court law clerks who were later Justices: Justice White (clerked for Chief Justice Vinson), Chief Justice Rehnquist (clerked for Justice Jackson), Justice Stevens (clerked for Justice Rutledge), and Justice Breyer (clerked for Justice Goldberg). A scan of the lists reflects that there is no overlap between the terms of service of any of these pairs of names."

Another instance on the circuit court level: Alex Kozinski clerked for Anthony Kennedy of the Ninth Circuit, and the two later served together (albeit in differen cities) from Kozinski's appointment to the same court in November 1985 until Kennedy was elevated to the Supreme Court in February 1988.